VDSL protocol with low power mode

ABSTRACT

A VDSL system is proposed which operates in a selected one of at least two modes. In one of those modes the volume of data that is to be transmitted is reduced. The invention proposes that this is done in such a way as to reduce the IFFT/FFT computational burden, to give a “low power” transmission mode (in contrast to the “high power” transmission mode in which all the tones are fully used). This also has the advantage that reduced memory is required, especially in the decoder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to the following co-pending and commonly assigned patent applications, all filed concurrently herewith: Ser. No. ______, entitled “Allocating Data Between Tones in a VDSL System” (attorney docket number 2005 LW 2383), Ser. No. ______, entitled “Computationally Efficient Protocols for VDSL System” (attorney docket number 2005 LW 2384), and Ser. No. ______, entitled “Trellis Modulation Protocols for a VDSL System” (attorney docket number 2005 LW 2386), which applications are hereby incorporated herein by reference.

This application claims priority to Singapore Patent Application 200401383-5, which was filed Mar. 5, 2004, and is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to methods for transmitting data, in particular over telephone lines (typically, copper telephone lines) or similar lines. It further relates to systems arranged to perform the methods.

BACKGROUND

The use of fast Internet connections has grown rapidly over the last few years, and consequently the demand for broadband (high-speed) connections is increasing.

One technology that is very well known in the market is Asymmetric Digital Subscriber Line (ADSL) technology. This employs the frequency spectrum indicated schematically in FIG. 1. “Upstream” communications (that is in the direction from the home or office user premises, “customer premises equipment” or “CPE”, to the “central office”, or “CO” or DSLAM, FTTC, or Fibre To The Curb, or FTTH, Fibre To The Home cabinets) are transmitted on frequencies in the range of 25 kHz (i.e., above the maximum audible frequency of 4 kHz) to 138 kHz. “Downstream” communications are in a higher frequency band from 138 kHz to an upper limit. According to the first two versions of ADSL (ADSL and ADSL2) the downstream band goes up to 1.1 MHz, whereas in ADSL2+ it goes up to 2.2 MHz. The upstream can be also extended from 0 khz up to 276 kHz, also known as All Digital Loop and extended upstream. Within each of the upstream and downstream bands, the range is divided into 4 kHz intervals, “tones,” so that the downstream band includes 256 tones in ADSL and ADSL2 (which is capable of transmitting 8 MBps), and 512 tones in ADSL2+ (which is capable of transmitting 28 MBps). Each tone is encoded by quadrature amplitude modulation (“QAM”), and can encode between 0 and 15 bits. During a training phase, the line conditions (signal to noise ratio, SNR) of each of the tones is estimated, and the number of bits which will be encoded in each tone during each frame is selected.

In a typical ADSL modem, the main sections are (i) a Digital Interface (which may use asynchronous transfer mode (ATM)); (ii) a Framer (also referred to here as a framing unit); (iii) an DMT Modulator; (iv) the AFE (Analog Front End); and (v) a Line Driver.

The framer multiplexes serial data into frames, generates FEC (forward error correction), and interleaves data. FEC and data interleaving corrects for burst errors. This allows DMT-based ADSL technology to be suitable for support of MPEG-2 and other digital video compression techniques. For the transmit signal, an Encoder encodes frames to produce the constellation data for the DMT Modulator. It assigns the maximum number of bits per tone (based on measured SNR of each tone) and generates a QAM constellation where each point represents a digital value. Each constellation point is one of N complex numbers, x+iy, where x and y are the phase and amplitude components. The summation of bits in all carriers, multiplied by the frame rate (4 kHz), represents the data rate. For the receive signal, the decoder converts QAM symbols back into the data bitstream.

In the DMT Modulator, a frequency domain processor implements FFT/IFFT and associated processing. In the transmit path, the Inverse Fast Fourier Transform (IFFT) module accepts input as a vector of N QAM constellation points and duplicates each carrier with its conjugate counterpart so the 2N output samples are real. The 2N time domain samples may have for example the last 2N/16 samples appended as a cyclic extension (which may include a cyclic suffix, a windowing function and/or a cyclic prefix extension) for every symbol, and are then delivered to a DAC (digital-to-analog converter). The set of time domain samples represents a summation of all the modulated sub-channels, for the duration of one data frame. In the receive path, the first 2N/16 samples (cyclic prefix) from the ADC are removed from every symbol. A FFT module transforms the carriers back to phase and amplitude information (N complex QAM symbols). Correction for attenuation of the signal amplitude and phase shifts (i.e., overall distortion) is implemented. If the QAM constellation is thought of as points in a grid where rows and columns represent phase and amplitude information respectively, then the grid effectively rotates reference to the constellation points to correct for these distortions.

Based on the SNR, which has been established for the tones, they are classified based on the SNR such that a “path” is selected for each tone through the encoding device, and each of the tones is transmitted along to the framing unit through the corresponding selected transmission path. This is illustrated in FIG. 2(a), in which the framing unit 1 for producing V/ADSL frames receives data along two paths 2, 3. Each path 2, 3 leads to a respective block 4, 5 which constructs respective portions of frames. The frame is shown in FIG. 2(b), including a portion 6 generated by block 8, and a portion 7 constructed by a block 9 (which may be an interleaver). The outputs of the blocks 4, 5 are stored respectively in a fast buffer 8 and interleaved buffer 9, until they are transmitted out of the framing unit 1. Since the interleaver 5 interleaves data over a period of time, data transmitted along path 3 will have a different (higher) latency than data transmitted along the path 2. Thus, these two paths are referred to as different “latency paths” (e.g. they may be referred to as LP1 and LP2). Note that both paths LP1 and LP2 may be interleaved.

DMT technology also includes a feature known as “tone ordering”. This means that the encoder, in forming VDSL symbols (there may be multiple VDSL frames within one VDSL symbol), determines the order in which subcarriers are assigned bits. The term tone ordering is wide enough to include both (i) determining the order in which the subcarriers are assigned data transmitted along a given latency path; and (ii) the order in which the subcarriers are assigned data transmitted along the different latency paths.

Furthermore, the number of bits that are transmitted by each of the tones may be modified if the estimated SNRs of the tones are revised: increasing the number of bits stored per frame in some tones and correspondingly reducing the number of bits stored per frame in other tones. There could be other reasons to dynamically change the bit allocation for spectral reasons too. This process is known as “bit swapping.”

For further details of the ADSL2 standard, the reader is referred to the document ITU-T Recommendation G.992.3 published by the International Telecommunication Union, the disclosure of which is incorporated herein by reference in its entirety.

SUMMARY OF THE INVENTION

Embodiments of the present invention aim to provide new and useful DSL protocols for transmitting data through lines such as telephone lines, and equipment for operating the protocols. Typically these protocols have maximum data transmission rates of over 24 Mbps, and often much higher.

In general terms, embodiments of the invention propose that a DSL system operate in a selected one of at least two modes. In one of those modes the volume of data that is to be transmitted is reduced. The invention proposes that this is done in such a way as to reduce the IFFT/FFT computational complexity, to give a “low power” transmission mode (in contrast to the “high power” transmission mode in which all the tones are fully used). This also has the advantage that reduced memory is required, especially in the decoder.

The two modes may for example correspond respectively VDSL communication to ADSL communication. More generally, the transition is from a first band plan to a second band plan, which employs a maximum frequency for data transmission that is lower than the maximum frequency used for data transmission in the first band plan. This may be generalized to greater than two modes, each with respective data transmission rates.

In one form of implementation of the IFFT/FFT computational burden (i.e., IFFT in the encoder, and FFT in the decoder), there may be a single IFFT/FFT module that normally employs an algorithm designed to be capable of processing data specifying the full range of tones available for transmission in the corresponding direction, but which can be switched to a mode of operation that uses the information that certain of the tones are not present to reduce the computation it performs.

Alternatively, it is possible to provide two separate modules for performing IFFT/FFT, one of which is adapted to perform an IFFT/FFT over the whole range of tones, and the other is adapted to perform an IFFT/FFT on a subset of the tones. Selecting a mode thus corresponds to selecting one of the two modules, e.g., selecting a respective path through the encoder/decoder.

In some forms of the invention the digital-to-analog converter and line driver can be controlled to change their mode of operation according to the frequencies that are output by the IFFT/FFT section of the apparatus.

Alternatively, when the number of frequency components is reduced (i.e., the IFFT/FFT processing employs fewer tones than the full number provided by the protocol), the maximum frequency may be increased by up-sampling. This aspect makes it possible for a single line driver and DAC (digital-to-analog converter) to be employed in both of the two modes, and for there to be a smooth transition from one mode to another.

When it is required to transfer between two (or more) operating modes, this may be done by a coordinated procedure in which one end of the communication path generates an instruction that the mode is to be changed, and this change of mode is then implemented at both ends, e.g., simultaneously, so that no data is lost.

An alternative is to affect the low power mode by the transmitter simply switching modes. In this case, during the transition period between the VDSL (or, more generally, first band plan) and ADSL (or more generally second band plan), and vice versa, data may perhaps be lost (and so may need to be retransmitted), but in certain implementations this cost may still be worthwhile for the power saving of ADSL.

Note that optionally the second band plan may include transmitting signals that do not encode data in frequencies above the maximum frequency it uses for data transmission. For example, signals that do not encode data (e.g., signals that are predefined) may be transmitted in the second band plan at all the frequencies above the maximum frequency that is used to transmit data in the second band plan and up to the maximum frequency that is used to transmit data in the first band plan. An advantage of transmitting such signals is that if this is not done the very fact that signals of these high frequencies are not being transmitted may mean that there are changes in the noise environment. Optionally, these frequencies may all be transmitted with the same gain, or more generally with a gain that is not chosen by an analysis of the SNRs of the individual tones in these frequencies. This reduces the memory that is required to store gains for each of the tones.

In two of the related applications referenced above, it is proposed that tones may be grouped (e.g., subsets of the tones may be defined; each of these subsets is composed only of tones that are used for transmission in the same direction). This grouping may have one of two functions. Firstly, the data that is to be transmitted using a given group of tones can be Trellis encoded together. Secondly, the group of tones can be used for one or more of (i) bit allocation; (ii) bit swapping; (iii) tone ordering; and/or (iv) gain allocation. The purpose of performing the five operations on groups of tones (rather than, for example, on all tones associated with data transmission in the same direction) is to reduce the computational and memory requirements of coding and decoding.

This concept may be combined with the present invention. For example, when there are two modes of operation, such that in a first mode all tones are used, and in a second mode only the tones below a certain cut-off frequency are used, the tones can be grouped in the first mode of operation such that no groups of tones span that certain frequency (i.e., include both at least one tone above the frequency and at least one tone at or below the cut-off frequency). For example, the tones up to and including the cut-off frequency could be grouped together (e.g., as groups of consecutive tones), and tones above the frequency could be grouped together (e.g., as groups of consecutive tones). Thus, the transition between the two modes means ceasing to transmit complete groups of tones, which minimizes the interference with the various uses of the groups described in the previous paragraph.

The invention, in its various aspects, may be expressed as a method of transmitting and/or receiving data (i.e., the method performed by the apparatus at any one end of the line, or the method performed by the system as a whole including the apparatus at both ends of the line). Alternatively, the invention may be expressed as the apparatus that performs either of those methods, and having means for implementing each of the method steps.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the invention will now be described, for the sake of illustration only, with reference to the following figures in which:

FIG. 1 shows the frequency usage of a conventional ADSL technique;

FIG. 2, which is composed of FIGS. 2(a) and 2(b), illustrates tone ordering in a conventional ADSL technique;

FIG. 3 shows the frequency usage of a conventional VDSL technique;

FIG. 4 shows schematically the structure of a first embodiment of the invention;

FIG. 5, which is composed of FIGS. 5(a) and 5(b), shows schematically the data transmission rate in different modes of the first embodiment of the invention;

FIG. 6 shows the implementation of the low power mode in a second embodiment of the invention;

FIG. 7 shows the implementation of the low power mode in a third embodiment of the invention;

FIG. 8, which is composed of FIGS. 8(a) to 8(f), shows possible band plans produced by either the first, second or third embodiments of the invention; and

FIG. 9, which is composed of FIGS. 9(a) to 9(c), shows grouping the tones in another embodiment of the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A protocol, which is a first embodiment of the present invention, will be described with reference to FIG. 4, which shows schematically the structure of a system capable of generating and transmitting signals according to this protocol. For simplicity many features of the protocol are not described in the following text, because they are generally according to the ADSL standard (which is described for example in the document ITU-T Recommendation G.992.3 published by the International Telecommunication Union, and is incorporated herein by reference).

As in conventional systems, the structure of FIG. 4 includes a framer 11, a unit 12 defining the fast path and performing the interleaving function, a QAM encoder 13, an IFFT unit 14, a filter 15, a digital-to-analog converter 16 and a line driver 17.

In the first embodiment of the invention, the protocol switches from a high data transmission rate mode (such as VDSL) referred to here as L₁, to a low data rate transmission mode (such as ADSL) referred to here as L₂. This is illustrated in FIG. 5(a).

Note that there may be more than two modes of operation, such that there is a choice of different power saving modes. This is illustrated in FIG. 5(b) in which the protocol uses a first power saving mode L₂ at a first time, and a second power saving mode L₄ at other times.

A change of mode involves a change in the operation of the IFFT 14 and a consequent change in the operation of the filter 14, DAC 16 and line driver 17. This change is performed by a processor (not shown) that controls all of the IFFT unit 14, filter 15, DAC 16 and line driver 17. The processor also controls the QAM encoder 13, so that it only encodes the data in the frequencies that will be used, and controls the framer unit 11 since data rate changes also impact the framer.

The first embodiment may, for example, be arranged to switch the mode of transmission from a VDSL mode of transmission to an ADSL mode of transmission. Hence, compared to the power consumption during the full traffic mode of VDSL, during the low power mode the power consumption will be much less, such as no more than a half of the former, or no more than a third of the former. Also it is suggested that the during the low traffic modes, the bands of transmission be limited to 2 k tones (8 Mhz spectrum) or 1 k tones (4 Mhz spectrum) likewise. The data in the remaining tones is all zeros. The FFT/IFFT does not have to process the samples with value zero, and some stages in the processing can be skipped or reduced significantly. Hence, the advantage is a reduction of the power usage during low-power mode. The power saving can be more than 70% depending on the modes that are used to carry the relevant data. Consequently, it lowers the heat dissipation requirements. This is particularly important for remote DSL equipment, where heat is a challenging problem.

A command message to switch to the low power mode is communicated by the ATU-C (ADSL termination unit—central office). Once the ATU-R (ADSL termination unit—remote) receives the message it may either accept the command or reject the same. If ATU-R in its acknowledgement sends an accept message, after a fixed duration coupled either with sync symbol, the ATU-C may change the state from VDSL to a low band mode (these modes of operation are named modes M0, M1, M2, M3, M4 or M5 in FIG. 8, which is described below). It will be a coordinated transition based on the command protocol, in which all of the IFFT 14, filter 15, DAC 16 and line driver 17 change their modes of operation. Since the ATU-C knows the end-to-end delay, it may either synchronize the transition to a new state after a fixed duration of a transmitted frame or could also use the synchronization frame as a reference. The ATU-C may also switch to a different mode of transmission after a fixed synchronization frame delay. A command message is appropriately defined to provide the information for the mode of transmission desired (the bands used, or limiting the FFT and frequency spectrum), and/or the delay after which the switch will happen. The frames with a fixed reference could be used to specify the delay.

Alternatively, the transmitter could be arranged to switch modes without this agreement occurring, and the receiver could be arranged to recognize that the mode of operation has changed. At the transmission points some data may be lost (since the device receiving the encoded data will in general not be aware that the protocol is being changed), so that retransmission of this data may be required. However, the cost of the retransmission may still be low in comparison to the power saving.

Above, the structure of the data transmission section of a bi-directional communication device has been described, but a skilled reader will readily understand the corresponding structures of the data reception section of the same apparatus. It will resemble known structures, except that a control processor is able to control the FFT section to operate in more than one mode, in a way exactly corresponding to the control of an IFFT section of a data transmission apparatus.

Similarly, a skilled reader will readily appreciate the construction of a data reception apparatus of the embodiments described below.

We now turn to a second embodiment of the invention shown in FIG. 6. The upper part of FIG. 6 illustrates a known structure for encoding and transmitting data in VDSL, and which performs the “high power” mode of operation of the embodiment. The output of the IFFT unit 20 is transmitted to a filter 21, which produces data having a frequency of 17 or 35 MHz depending upon the sampling rate. The output of the filter 21 is passed to a DAC 22, from there to a line driver 23, and from there to a transformer 24, which increases the voltage for transmission onto the line.

In the embodiment of FIG. 6, the low power mode of operation is performed by a second signal path, which bypasses the IFFT unit 20 (thus considerably saving power) and instead employing an IFFT unit 30 such as used conventionally in a ADSL system. That is one, which is able to receive fewer tones than the unit 20, preferably only about 1/16^(th) of the number of tones. This may be 128 or 256 tones, for example. The output of the IFFT 21 is transmitted to an up-sampler 31 which up-samples the frequency, e.g., by a factor of 16. The output of the up-sampler 31 is transmitted to the filter 21. The power consumption of the path of the lower part of FIG. 8 is considerably less than that of the IFFT unit 20, and thus there is a considerable power saving, but this does not disrupt the operation of the units 21 to 25 which receive (as far as they are concerned) equivalent input signals. Thus, in contrast to the first embodiment, the modes of operation of the DAC 22 and line driver 23 do not have to be altered.

FIG. 7 shows an example of a possible structure of a third embodiment of the invention. Units having the same meanings as those of FIG. 4 are given the same reference numerals. As in the first embodiment (and unlike the second embodiment) the IFFT unit 14 has at least two modes of operation. In contrast to FIG. 4, in the third embodiment of FIG. 7 in the low power mode the transmission is limited to 256 tones but maintains the data rate on the interface. To ensure the same a simple (16×) interpolator 18 is present after the IFFT 14. It is active only when the IFFT 14 is in its low power mode. The interpolator 18 could assist to bring the frequency back to the desired 17 Mh spectrum. Other kinds of circuitry could alternatively be used. The interpolator 18 could be located either after the filter 15 or before the filter. Hence, significant power savings could be achieved for the frequency domain processing. The above shows the scenario for transmit but the same is also valid for the receive direction.

FIG. 8 shows band plans that can be produced in the various modes of operation of any of the embodiments of the invention. FIG. 8(a) shows the full frequency range (up to 17 MHz) being used for transmitting data (i.e., the VDSL high power mode).

FIG. 8(b) shows a different mode of operation (“M1”), in which only frequencies up to 1.1 MHz are used for transmitting data. Those frequencies above 1.1 MHz are shown with low values to indicate that no signal is transmitted on those frequencies, or (in other forms of the embodiments) that signals are transmitted that do not carry data. For example, it may be advantageous to broadcast signals that do not carry data on frequencies 1.1 MHz to 17 MHz, to avoid changing the noise environment.

Note that to produce this mode the embodiment of FIG. 4 would require some additional adaptations to the DAC module and filter module to transmit data on higher frequencies.

In yet further versions, the maximum transmission rate may be lower for the range of frequencies that are shown as having a low value. In other words, the range of frequencies (a “high frequency range”) between 1.1 MHz and 17 MHz may be used to carry a certain data load in the first mode of operation, and a lower amount of data in the mode of FIG. 8(b).

FIG. 8(c) shows a third mode of operation (“M2”) in which frequencies up to 12 MHz are used for transmitting data. Just as described above in relation to FIG. 8(a), the frequencies above 12 MHz, which are shown as “low” may be unused, used for non-data transmission, or used for data transmission at a lower data transmission rate than in FIG. 8(a).

FIG. 8(d) shows a fourth mode of operation (“M3”) in which frequencies up to 8 MHz are used for transmitting data. Just as described above in relation to FIG. 8(a), the frequencies above 8 MHz, which are shown as “low” may be unused, used for non-data transmission, or used for data transmission at a lower data transmission rate than in FIG. 8(a).

FIG. 8(e) shows a fifth mode of operation (“M4”) in which frequencies up to 5.3 MHz are used for transmitting data. Just as described above in relation to FIG. 8(a), the frequencies above 5.3 MHz, which are shown as “low” may be unused, used for non-data transmission, or used for data transmission at a lower data transmission rate than in FIG. 8(a).

FIG. 8(f) shows a sixth mode of operation (“M5”) in which frequencies up to 4.4 MHz are used for transmitting data. Just as described above in relation to FIG. 8(a), the frequencies above 4.4 MHz, which are shown as “low” may be unused, used for non-data transmission, or used for data transmission at a lower data transmission rate than in FIG. 8(a).

Note that in all the examples of FIG. 8 the low data rate mode applies to both the upstream and downstream directions. This is the typical case, since Tx and Rx are coupled. However, the invention is not limited in this respect, and certain embodiments of the invention make possible transmission at the maximum data rate in one direction and at a lower data rate in the other direction.

In two of the related applications referenced above, it is proposed that tones may be grouped (e.g., subsets of the tones may be defined; each of these subsets is composed only of tones that are used for transmission in the same direction). This grouping may have one of two functions. Firstly, the data that is to be transmitted using a given group of tones can be Trellis encoded together. Secondly, the group of tones can be used for one of more of (i) bit allocation; (ii) bit swapping; (iii) tone ordering; and/or (iv) gain allocation. The purpose of performing the five operations on groups of tones (rather than, for example, on all tones associated with data transmission in the same direction) is to reduce the computational and memory requirements of coding and decoding.

This concept may be combined with the present invention. For example, suppose that in the protocol the band scheme includes the bands shown in FIG. 9(a), that is with two upstream bands marked US1 and US2 and two downstream bands marked DS1 and DS2. Suppose further that the tones of the downstream bands are as shown in FIG. 9(b)—divided into groups GD1, GD2, GD3 GD4, GD5 and GD6, and that the tones of the upstream bands are groups as shown in FIG. 9(c)—i.e., divided into groups GU1, GU2, GU3, GU4, GU5 and GU6. Thus, it is possible to implement the present invention by ceasing to transmit data at all frequencies above any of the boundaries between two of the groups of tones.

Although only a few embodiments of the invention have been disclosed in this application, many variations are possible within the scope of the invention as will be clear to a skilled reader. 

1. A method of two-directional communication of data over a line, the communication employing a bandwidth partitioned into a number of bands including at least one band associated with each of the two directions, the method including encoding the data carried in each direction by modulation of a plurality of tones defined within the at least one band associated with that direction, the transmission in each direction being implemented by: generating amplitude data indicating complex amplitudes of respective tones associated with the corresponding direction; and performing an IFFT transform of the amplitude data to obtain a time-domain signal for transmission in the corresponding direction; wherein the IFFT transform is performed in a selected one of at least two modes: in a first said mode, the IFFT transform being performed for amplitude data describing the amplitudes of all the tones associated with the corresponding direction; and in a second said mode, the IFFT transform being performed for amplitude data describing the amplitudes of only a subset of the tones associated with the corresponding direction.
 2. The method according to claim 1 wherein, for each direction, the tone of highest frequency associated with that direction has a frequency above that of the highest frequency tone in the corresponding subset of tones.
 3. The method according to claim 2 wherein the subset of tones is composed of all the tones associated with the corresponding direction and having a frequency below a respective predetermined cut-off frequency.
 4. The method according to claim 1 wherein: in the first mode the IFFT transform is performed using a first IFFT unit which receives amplitude data describing the amplitudes of all the respective tones associated with the corresponding direction; and in the second mode, the IFFT transform is performed using a second IFFT unit which receives only data describing the amplitudes of the subset of tones.
 5. The method according to claim 1 wherein, in both the first and second modes, the IFFT is performed using a single IFFT unit, the IFFT unit being controlled to operate in the first or second mode by a control signal, in the second mode the IFFT unit performing an IFFT transform only of amplitude data determining the respective amplitudes of the subset of tones.
 6. The method according to claim 1 wherein an output of the IFFT transform is transmitted using a digital-to-analog converter which receives the output of the IFFT transform, and a line driver which receives the output of the digital-to-analog converter.
 7. The method according to claim 6 wherein, in the second mode, the operation of the digital-to-analog converter and the line driver is modified.
 8. The method according to claim 6 wherein, in the second mode, the output of the IFFT transform is up-sampled to increase its maximum frequency.
 9. The method according to claim 1 wherein the second mode of operation is a mode that complies with an ADSL standard.
 10. The method according to claim 1 wherein a transition between the two modes occurs at times which are indicated in advance by signals transmitted along the line.
 11. A data transmission apparatus for transmitting data as part of a process of two-directional communication of data over a line, the communication employing a bandwidth partitioned into a number of bands including at least one band associated with each of the two directions, the data transmission apparatus including: a signal generation section for generating a signal encoding the data to be transmitted along the line in a first of the two directions, by modulation of a plurality of tones defined within the at least one band associated with the first direction, the signal generation section including: an input section for receiving data; an encoder for generating amplitude data indicating complex amplitudes of respective tones associated with the first direction; an IFFT transform section performing an IFFT transform of the amplitude data to obtain a time-domain signal for transmission in the first direction; and a processor for controlling the IFFT transform section; and a signal transmission section including a line driver for transmitting the time-domain signal along the line in the first direction, wherein the processor is arranged to control the IFFT transform section to operate in a selected one of at least two modes: in a first said mode, the IFFT transform section performing an IFFT transform of amplitude data describing the amplitudes of all the tones associated with the first direction; and in a second said mode, the IFFT transform performing an IFFT transform of amplitude data describing the amplitudes of only a subset of the tones associated with the first direction.
 12. The data transmission apparatus according to claim 11 wherein the tone of highest frequency associated with the first direction has a frequency above that of the highest frequency tone in said subset of tones.
 13. The data transmission apparatus according to claim 12 wherein the subset of tones is composed of all the tones associated with the first direction and having a frequency below a predetermined cut-off frequency.
 14. The data transmission apparatus according to claim 11 wherein the IFFT transform section includes: a first IFFT unit which, in the first mode, receives amplitude data describing the amplitudes of all the respective tones associated with the corresponding direction, and a second IFFT which, in the second mode, receives data describing the amplitudes of the subset of tones.
 15. The data transmission apparatus according to claim 11 wherein the IFFT transform section includes a single IFFT unit operative under control of the processor selectively to perform an IFFT transform of amplitude data for all the tones associated with the first direction, or of amplitude data for only the subset of tones.
 16. The data transmission apparatus according to claim 11 wherein the signal transmission section includes a digital-to-analog converter that receives the output of the IFFT transform section, and a line driver that receives the output of the digital-to-analog converter.
 17. The data transmission apparatus according to claim 16 wherein the processor controls the digital-to-analog converter and line driver according to whether the IFFT transform section is operating in the first or second mode.
 18. The data transmission apparatus according to claim 17 wherein the IFFT transform section includes an up-sampler to increase the maximum frequency of the output of the IFFT transform section.
 19. The data transmission apparatus according to claim 11 wherein the apparatus is operative to generate data according to an ADSL protocol when operating in said second mode of operation.
 20. The data transmission apparatus according to claim 11 wherein the processor is operative to communicate with a remote processor located at the other end of the line to determine a time for transition between the modes.
 21. A data reception apparatus for receiving data as part of a process of two-directional communication of data over a line, the communication employing a bandwidth partitioned into a number of bands including at least one band associated with each of the two directions, the data reception apparatus including: a signal reception section for receiving a signal transmitted along the line in a first direction and encoding data by modulation of a plurality of tones defined within the at least one band associated with the first direction; and a decoding section comprising: an FFT transform section performing an FFT transform of the signal to obtain frequency-domain amplitude data; a decoder for decoding the frequency-domain data; and a processor for controlling the FFT transform section; wherein the processor is arranged to control the FFT transform section to operate in a selected one of at least two modes: in a first said mode, the FFT transform section performing an FFT to derive amplitude data describing the amplitudes of all the tones associated with the first direction; and in a second said mode, the FFT transform section performing an FFT to derive amplitude data describing the amplitudes of only a subset of the tones associated with the first direction. 